Move gdal test out of workflow

[maresb]

I'd like to get rid of this:

      - name: test-gdal
        shell: bash -l {0}
        run: |
          conda activate test
          pushd "${RUNNER_TEMP}/tests/gdal"
          export TMPDIR="${RUNNER_TEMP}"
          ls -lah
          conda-lock --mamba  -f environment.yml -p linux-64

It runs at the end of the test workflow for non-windows runners and installs the gdal package. Locally it takes 37 seconds to run. In contrast, an environment with Python-only takes 27 seconds, and ibis-framework takes 1:27.

For the purpose of cleaning up random cruft, I'd like to either remove it or move it into pytest. (Given the speed of Mamba these days, I don't even think it would be so bad to run it on Windows, or we could add a mark.)